Spring Boot Interview Questions

The advantages of Spring Boot are listed below:

• Easy to understand and develop spring applications.
• Spring Boot is nothing but an existing framework with the addition of an embedded HTTP server and annotation configuration which makes it easier to understand and faster the process of development.
• Increases productivity and reduces development time.
• Minimum configuration.
• We don’t need to write any XML configuration, only a few annotations are required to do the configuration.

Below are the four key components of spring-boot:

• Spring Boot auto-configuration.
• Spring Boot CLI.
• Spring Boot starter POMs.
• Spring Boot Actuators.

Below are some key points which spring boot offers but spring doesn’t:

• Starter POM.
• Version Management.
• Auto Configuration.
• Component Scanning.
• Embedded server.
• InMemory DB.
• Actuators

Spring Boot simplifies the spring feature for the user:

Spring boot provides numbers of starter dependency, here are the most commonly used -

• Data JPA starter.
• Test Starter.
• Security starter.
• Web starter.
• Mail starter.
• Thymeleaf starter.

Spring Boot automatically configures your application based on the dependencies you have added to the project by using annotation. The entry point of the spring boot application is the class that contains @SpringBootApplication annotation and the main method.

Spring Boot automatically scans all the components included in the project by using @ComponentScan annotation.

The @SpringBootApplication annotation is equivalent to using @Configuration, @EnableAutoConfiguration, and @ComponentScan with their default attributes. Spring Boot enables the developer to use a single annotation instead of using multiple. But, as we know, Spring provided loosely coupled features that we can use for each annotation as per our project needs.

Spring Boot application scans all the beans and package declarations when the application initializes. You need to add the @ComponentScan annotation for your class file to scan your components added to your project.

Just like any other Java program, a Spring Boot application must have a main method. This method serves as an entry point, which invokes the SpringApplication#run method to bootstrap the application.

@SpringBootApplication 
public class MyApplication { 
   
       public static void main(String[] args) {    
    
             SpringApplication.run(MyApplication.class);        
               // other statements     
       } 
}

Spring boot starter is a maven template that contains a collection of all the relevant transitive dependencies that are needed to start a particular functionality.
Like we need to import spring-boot-starter-web dependency for creating a web application.

<dependency>
<groupId> org.springframework.boot</groupId>
<artifactId> spring-boot-starter-web </artifactId>
</dependency>

Spring Initializer is a web application that helps you to create an initial spring boot project structure and provides a maven or gradle file to build your code. It solves the problem of setting up a framework when you are starting a project from scratch.

Spring Boot CLI is a command-line interface that allows you to create a spring-based java application using Groovy.

Example: You don’t need to create getter and setter method or access modifier, return statement. If you use the JDBC template, it automatically loads for you.

-run, -test, -grap, -jar, -war, -install, -uninstall, --init, -shell, -help.

To check the description, run spring --help from the terminal.

The primary annotations that Spring Boot offers reside in its org.springframework.boot.autoconfigure and its sub-packages. Here are a couple of basic ones:

@EnableAutoConfiguration – to make Spring Boot look for auto-configuration beans on its classpath and automatically apply them.

@SpringBootApplication – used to denote the main class of a Boot Application. This annotation combines @Configuration, @EnableAutoConfiguration, and @ComponentScan annotations with their default attributes.

Spring Boot dependency management is used to manage dependencies and configuration automatically without you specifying the version for any of that dependencies.

Yes, we can create a non-web application by removing the web dependencies from the classpath along with changing the way Spring Boot creates the application context.

Yes, it is possible. By using the server.port in the application.properties.

The default port of the tomcat server-id 8080. It can be changed by adding sever.port properties in the application.property file.

Yes, we can replace the Embedded Tomcat server with any server by using the Starter dependency in the pom.xml file. Like you can use spring-boot-starter-jetty as a dependency for using a jetty server in your project.

Yes, we can use application.properties to configure the web application type i.e spring.main.web-application-type=none.

You can use exclude attribute of @EnableAutoConfiguration if you want auto-configuration not to apply to any specific class.

//use of exclude
@EnableAutoConfiguration(exclude={className})

It is a combination of @Controller and @ResponseBody, used for creating a restful controller. It converts the response to JSON or XML. It ensures that data returned by each method will be written straight into the response body instead of returning a template.

@Controller Map of the model object to view or template and make it human readable but @RestController simply returns the object and object data is directly written in HTTP response as JSON or XML.

On a high-level spring boot application follow the MVC pattern which is depicted in the below flow diagram.

RequestMapping can be used with GET, POST, PUT, and many other request methods using the method attribute on the annotation. Whereas getMapping is only an extension of RequestMapping which helps you to improve on clarity on request.

While developing the application we deal with multiple environments such as dev, QA, Prod, and each environment requires a different configuration. For eg., we might be using an embedded H2 database for dev but for prod, we might have proprietary Oracle or DB2. Even if DBMS is the same across the environment, the URLs will be different.

To make this easy and clean, Spring has the provision of Profiles to keep the separate configuration of environments.

An actuator is an additional feature of Spring that helps you to monitor and manage your application when you push it to production. These actuators include auditing, health, CPU usage, HTTP hits, and metric gathering, and many more that are automatically applied to your application.

To enable the spring actuator feature, we need to add the dependency of “spring-boot-starter-actuator” in pom.xml.

<dependency>
<groupId> org.springframework.boot</groupId>
<artifactId> spring-boot-starter-actuator </artifactId>
</dependency>

Actuators provide below pre-defined endpoints to monitor our application -

• Health
• Info
• Beans
• Mappings
• Configprops
• Httptrace
• Heapdump
• Threaddump
• Shutdown

Spring Boot actuator “/Beans” is used to get the list of all the spring beans in your application.

Spring Boot actuator “/env” returns the list of all the environment properties of running the spring boot application.

Debugging logs can be enabled in three ways -

• We can start the application with --debug switch.
• We can set the logging.level.root=debug property in application.property file.
• We can set the logging level of the root logger to debug in the supplied logging configuration file.

You can define both application and Spring boot-related properties into a file called application.properties. You can create this file manually or use Spring Initializer to create this file. You don’t need to do any special configuration to instruct Spring Boot to load this file, If it exists in classpath then spring boot automatically loads it and configure itself and the application code accordingly.

The process of injecting dependent bean objects into target bean objects is called dependency injection.

• Setter Injection: The IOC container will inject the dependent bean object into the target bean object by calling the setter method.
• Constructor Injection: The IOC container will inject the dependent bean object into the target bean object by calling the target bean constructor.
• Field Injection: The IOC container will inject the dependent bean object into the target bean object by Reflection API.

IoC Container is a framework for implementing automatic dependency injection. It manages object creation and its life-time and also injects dependencies into the class.

Important Resources

• Java Spring Boot Free Course with Certificate
• Spring vs Spring Boot
• Difference Between Spring MVC and Spring Boot
• Spring Interview Questions
• Spring Security Interview Questions